1. Field of the Invention
The present invention relates generally to improvements in hard disc drives and, more particularly but not by way of limitation, to improvements in methods and apparatus for acquiring and following data tracks defined on discs of a hard disc drive.
2. Brief Description of the Prior Art
In recent years, hard disc drives have increasingly become the device of choice for providing long term memory for small computers. Such drives generally provide a large data storage capacity within a small volume that facilitates mounting of the hard disc drive within the cabinet of the computer so that the computer can be constructed as a self-contained unit which provides the user with both the means for generating files and an on board storage capacity that is suitable for any purpose for which he may have purchased the computer. Moreover, modern hard disc drives are generally constructed to provide ready and rapid access to files that may have been previously stored or equally ready and rapid access to unused data storage locations for the storage of files the user creates. Indeed, storage capacities are currently expressed in gigabytes and average data track access times are expressed in milliseconds.
The large data capacity of a hard disc drive is achieved in several ways. Thus, for example, a hard disc drive will typically be comprised of a plurality of data storage discs having magnetizable surface coatings to which data files are written and a corresponding plurality of transducer heads that effect the writing and subsequent reading of the files. One approach to increasing data storage capacity is to increase the number of data storage discs included in the hard disc drive and an example of this approach is U.S. patent application Ser. No. 639,136 entitled "Apparatus and Method for Mounting Transducer Heads in a Hard Disk Drive" filed Jan. 9, 1991 by Cain et al. Such approach, however, will, in many cases, be limited by space requirements placed on a hard disc drive for mounting in a computer cabinet.
A second approach to increasing the data storage capacity of a hard disc drive is illustrated by recent improvements in the packing of data on the discs. In a hard disc drive, files are written to concentric data tracks that are defined on the surfaces of the discs and it has been recognized that the capacity of a disc drive can be maximized, consistently with minimization of errors upon writing and subsequent reading of the data, by writing the data at different rates at different radii of the heads on the disc. An example of this approach to data storage capacity maximization is described in U.S. Pat. No. 4,799,112, issued Jan. 17, 1989 to Bremmer et al.
A third approach to increasing disc drive data storage capacity, that with which the present invention is concerned, is to minimize the radial spacing of the data tracks. Such minimization is achieved by providing the hard disc drive with a servo system that reads servo information from the discs and continually repositions the locations of the transducer heads with respect to the data tracks on the discs.
In general, two types of servo systems are used. In an "embedded" system, the servo patterns are embedded in the data tracks so that a direct approach to track following is used. The problem with this direct approach is that it is wasteful of data storage capacity; either a large portion of the disc surfaces must be allocated to the servo system, to provide frequent sampling of the transducer head locations, or the data track spacing must be made large enough that any inaccuracy in the alignment between a transducer head and a track the head is following is within acceptable limits.
In the second type of servo system, an indirect approach to data track following is employed. One surface of one disc, a dedicated servo surface, is used solely for servo purposes and is written, during manufacture of the disc drive, with a servo pattern that repeats both radially and circumferentially across the dedicated servo surface. However, problems then arise with respect to alignment both of the servo head that reads the servo pattern and read/write heads that read and write data and with respect to the discs themselves. Without compensation for these misalignments, the data tracks must again be separated sufficiently that the read/write heads will follow the data tracks within acceptable limits.
Servo systems are further classified in accordance with the manner in which the servo circuit is realized. In an analog servo system, correction signals provided to a power amplifier that drives the actuator are developed, as the name indicates, in analog circuitry provided for the purpose An advantage of analog systems is that the servo pattern, which is digitally sampled, can be sampled at a rapid rate that enables close control of the transducer head locations during track following. A disadvantage is that the structure of an analog servo system is not readily modified so that limitations are placed on the incorporation of new developments; for example, adaptive track following techniques, in the disc drive art.
An alternative type of servo system is a digital servo system in which the sampled values are inputted to a microcomputer that calculates and outputs the correction signals. Such systems are readily modified, via re-progamming of the microcomputer, so that digital servo systems have a potential for inclusion of substantially any technique that might permit higher track densities to be achieved in a hard disc drive.
Unfortunately, this potential has not been realized in prior art digital servo systems. Additional programming required, for example, to incorporate adaptive techniques into the servo system, can slow the operation of the microcomputer during track following to increase sampling times so that any increase in track density that might be gained by adopting a technique might be lost through an increase in servo pattern sampling time.